1. Field of the Invention
The present invention relates to a boiler scale collecting device for collecting solid particulates (boiler scales) in a main steam pipe extending from a boiler to a power generation steam turbine in a power plant for preventing erosion of a steam turbine blade.
2. Description of the Prior Art
In a thermal power plant or the like, steam oxide scales are often generated on boiler tube materials, which peel due to quick starts or sudden load changes and cause erosion damage of a steam turbine blade with the peeled scales. Thus, countermeasures for preventing such erosion damage have been long desired. As such the countermeasures, the following have been practiced:
1. As a countermeasure on the steam turbine side, the blade is hardened for enhancement of erosion resistance. But this has only a short life and is expensive and allopathic. Thus there is a need to replace the blade at every periodic repair, which results in a largely elongated work period for the blade surface hardening treatment. Also, if there is a large amount of scale carried, damage may arise within a short period, requiring needs a severe maintenance control. Further, the blade surface hardening treatment cannot be measured on site, which makes life evaluation thereof difficult.
2. As a countermeasure on the boiler side, there is used a high class of material or boiler tube material on which boiler steam oxide scales are hardly generated, or accumulated scales are removed by cutting a boiler tube. But employment of this measure is actually difficult because of the cost and work period, and because of there being no fundamental countermeasure. Also, chemical cleaning is used for removing scales on an inner surface of a main steam pipe using chemicals, but this is expensive, its efficiency is low and there is uncertainty in the effect when this cleaning is used alone. Moreover, this cleaning is also allopathic and requires periodic performance.
3. A geothermal steam pipe extending to a steam turbine has a bent portion where a horizontal portion is turned to a vertical portion near an inlet of the steam turbine. The horizontal portion is elongated to form an elongated portion and a scale collecting mesh screen is provided in the elongated portion via a bolt type flange coupling (Japanese laid-open utility model application No. Sho 61-22517). But has this device the following problem:
(1) Geothermal steam to be used is of a low steam pressure of 10 kg/cm.sup.2 or so in the steam pipe, and the device is constructed such that the scale collecting portion is uncoupled for discharge of the collected scales. Hence the device is not applicable to a high temperature and high pressure steam pipe in a thermal power plant boiler in terms of strength and sealing ability. PA1 (2) The scale collecting portion has a grid screen in front thereof on which the scales impinge, with resultant re-scattering thereof. PA1 (3) The scale collecting portion is disposed vertically, which makes the steam density high when the steam is of high temperature and high pressure as compared with the geothermal steam with a resultant small density difference between the steam and a the scales, and a scale collecting performance, if the scale collecting is to be done only by inertia force, is reduced. PA1 (1) The scales are prevented by the guide plate from flowing into the scale collecting portion by the inertia force with resultant re-scattering thereof. PA1 (2) The guide plate and the magnet are disposed in the steam pipe in which the steam flows at a high velocity of 50 m/s or more, with a resultant problem of resonance, which is to be avoided. PA1 (3) The device is constructed such that a flat plate flange coupling having the magnet is uncoupled for discharge of the collected scales, hence the device is not applicable to a high temperature and high pressure use in terms of strength and sealing ability.
4. A scale collector has a guide plate and a magnet in a horizontal end portion of a steam pipe (Japanese laid-open patent application No. Sho 60-169100). But this device has the following problem:
5. As steam pressure in a steam pipe extending from a boiler to a steam turbine inlet is as high as 40 to 245 kg/cm.sup.2, while a side face of a vertical portion of the steam pipe and a lateral portion thereof may be connected together by welding, it requires skill completely uniform welding is not always possible and the strength thereof may deteriorated. Hence there is a problem in the practical use.
Also, when the steam turns from the vertical portion to the lateral portion, while boiler scale particulates want to go down toward a scale collecting portion due to inertia force in the vertical direction and gravity (G), there are upward flows, swirling flows and the like caused by the turning of the steam in the scale collecting portion by which the boiler scale particulates scatter and are entrained toward the lateral portion, resulting in a possibility of flowing into the steam turbine.
Entrainment of the scales into the steam turning flow is decided by the quantity and particle sizes of the boiler scale particulates in the steam and a depth of the scale collecting portion. As the result of actual state simulation tests, however, it has been found that if the operation is within a predetermined legal periodical inspection period (usually two years), the entrainment of the scales may be prevented only by the depth of the scale collecting portion being set to a predetermined value or more relative to a steam pipe inner diameter, regardless of the quantity and particle sizes of the boiler scale particulates.
6. Japanese laid-open patent application No. Hei 8-28208 discloses a steam turbine blade erosion preventing device for preventing the blade from being eroded by solid particulates. The device is constructed such that a main steam pipe extending from a boiler side to a steam turbine inlet is turned in direction near the steam turbine inlet, a scale collecting portion is provided right below the direction turning portion of the main steam pipe and a scale discharge valve is provided at a lower end of the scale collecting portion. In this device, however, there arise swirling flows of the steam in a pocket according to its depth to cause re-scattering of the collected scales. Hence by the re-scattering of the scales and the swirling flows, the pocket interior may be eroded.
As one example of such a device in the prior art, there is a scale collecting device as shown in FIG. 12. A steam pipe has a bent portion so that a steam main flow 004 may turn quickly from a vertical downward pipe 001 to a horizontal branch pipe 002, a tubular pocket 003 for collecting scales is provided at a lower portion of the vertical downward pipe 001, whereby scales 005 in the steam are separated by the inertia force and the centrifugal force caused by the quick turning of the steam main flow 004.
In the scale collecting device having the steam piping shown in FIG. 12, turbulence of the steam main flow 004 flowing from the vertical downward pipe 001 to the horizontal branch pipe 002 arises, whereby unsteady flow 004' in the tubular pocket 003 swirls around a central axis of the tubular pocket 003 so that the scales 005 which have been once separated move violently in the tubular pocket 003 to again mix into the main flow. This not only results in a lowering of the collection efficiency, but also is a cause of erosion of a bottom portion of the tubular pocket 003, because the scales 005 impinge thereon intermittently.